An offshore oil and gas well in Australia leaked oil continuously into the ocean for two months in 2016, releasing an estimated 10,500 litres. But the spill was never made public by the regulator and details about the well, its whereabouts and operator remain secret.

After inquiries from the Guardian, Nopsema said the leak went on for two months, at a rate of about 175 litres a day. It went unnoticed while the floating platform was undergoing maintenance and was only discovered when the platform returned.

A spokesman for Nopsema said the leak had been caused by a seal degrading. The regulator investigated the spill and said the operator had been ordered to check the seals were working before disconnecting the platform.

But despite requests to reveal exactly where the spill occurred, or what company was responsible, Nopsema refused to disclose the information, revealing only that it was in the North West Shelf.

The Nopsema spokesman said that since companies were compelled by law to report these leaks the regulator believed there was an “implied duty of confidence”.